We intend to study factors affecting changes in transmission through a reflex circuit in the mammalian central nervous system. The reflex chosen for investigation is the plantar cushion (PC) reflex, a cutaneous, spinal reflex in the cat. The neural circuit for the PC reflex appears to consist of not more than two serially connected populations of dorsal horn cells,, interposed between the cutaneous afferents from the PC and the montoneurons to the intrinsic plantar muscles. Neurophysiological analyses, employing principally intracellular recordings, will be undertaken to investigate changes that may occur in afferent fibers, dorsal horn cells and motoneurons during bouts of stimulation, at 1.0 to 10 Hz, that produce reflex sensitization and habituation. The possibility of classically conditioning the PC reflex will be further investigated. If this form of learning can be convincingly demonstrated, we intend to analyze physiological properties and responses of neuronal elements of the PC reflex the formation of such conditioning.